Flooding threatens US coasts well ahead of Hurricane Matthew

The major threat to the Southeast U.S. will be water  as the deadly storm surge churned up by such a massive and powerful hurricane.

The major threat to the Southeast U.S. won't be Hurricane Matthew's pummeling winds, which newer buildings have been constructed to withstand. It will be water — the deadly storm surge churned up by such a massive and powerful hurricane.

That surge could threaten lives and property long before its eye nears shore, so the Miami-based National Hurricane Center has issued experimental storm surge watches and warnings for life-threatening flooding for some 500 miles of coastline from Boca Raton in South Florida all the way up into North Carolina. Matthew already has left more than 100 dead in the Caribbean.

Storm surge and rainfall flooding have combined for three-quarters of U.S. deaths from hurricanes, tropical storms and cyclones over the last half-century — including at least 1,500 deaths during Hurricane Katrina, according to the hurricane center.

These prototype watches and warnings are among forecasting changes made after Superstorm Sandy revealed how often the public failed to understand the flooding risks from tropical storms. Forecasters changed their vocabulary to explain how flooding can occur far from the coastline, and they now publish interactive graphics illustrating risks from the ocean as well as sounds, bays and lakes.

Water levels were rising as far north as Jacksonville on Thursday morning as a strengthening Matthew tore through the Bahamas toward Florida, said Jamie Rhome, leader of the hurricane center's storm surge team.

"The ocean's reaction to the hurricane extends well in front of it, and that catches people off guard," Rhome said.

Rhome said parts of Florida, such as the Cape Canaveral area or communities along the St. Johns River, could see waters rise up to 9 feet above ground — a level well overhead for most adults.

"At those levels it becomes life-threatening because it's not a gentle rising of the water. It comes with waves and currents and floating debris," Rhome said.

When Hurricane Jeanne hit Florida's Space Coast in 2004, it caused storm surge flooding up to 6 feet above normal tides from Melbourne south to Fort Pierce. However, that area still has many older homes built before the National Flood Insurance Program included waves and storm surge in its elevation requirements, said Tim Reinhold, head of research and chief engineer at the Insurance Institute for Business and Home Safety.

New construction along the densely populated coastline includes support for high-rise buildings even if water pours through their ground floors, Reinhold said.

"Older buildings are at the greatest risk because they weren't built all that well and they were built at lower elevations. They haven't been weeded out in that area," Reinhold said.

In simple terms, storm surge is the abnormal rise of sea water. However, it's so complicated to predict that forecasters removed flooding references in a 2010 revision of the five-category system for describing the damage possible at various hurricane wind speeds.

Earlier this year, Hurricane Hermine was the first time forecasters experimented with storm surge watches and warnings for the public. The advisories, which are separate from watches and warnings about a storm's wind speeds, may become fully operational next year.

The storm surge threat only increased over the last decade, as the coastal population from North Carolina to Texas grew 13 percent to 27 million, according to U.S. Census estimates.

From Maine to Texas, the Census estimates there are 60.1 million housing units and 3.3 million business establishments with 52.3 million paid workers potentially at risk during the six-month Atlantic storm season. Much of that coastline lies less than 10 feet above mean sea level, according to the hurricane center.